In recent years, enhancement of fuel economy of the automobile has become an important issue from the viewpoint of global environmental conservation. Consequently, there is an active movement afoot to reduce the thickness through increases in strength of car body materials to reduce the weight of a car body itself.
However, an increase in strength of a steel sheet causes reduction in ductility, that is, reduction in formability. Therefore, development of materials having high strength and high formability in combination has been desired.
Furthermore, in forming of the high strength steel sheet into a complicated shape, e.g., an automobile component, occurrences of cracking and necking in a punch stretch portion or a stretch flange portion cause serious issues. Therefore, a high strength steel sheet which can overcome the issues on occurrences of cracking and necking and which has high ductility and high hole expansion property in combination has also been required.
Moreover, shape fixability is degraded by an increase in strength and thickness reduction of a steel sheet significantly. To cope with this, in press forming, it has been widely performed that changes in shape after release from a mold is predicted and the mold is designed in expectation of the amount of change in shape. However, if the tensile strength (TS) of a steel sheet is changed, deviation from the expected amount, in which the tensile strength is assumed to be constant, becomes large and odd shapes occur. Consequently, reworking, e.g., sheet-metal working of the shape on a one-by-one basis, becomes necessary after press-forming, and efficiency in mass production is degraded significantly. Therefore, it is required that variations in TS of the steel sheet are minimized.
As for an improvement of formability of the high strength steel sheet, heretofore, various multi phase high strength galvanized steel sheets, e.g., a ferrite-martensite dual-phase steel and a TRIP steel taking advantage of the transformation induced plasticity of retained austenite, have been developed.
For example, Japanese Unexamined Patent Application Publication No. 2001-140022 discloses a method of manufacturing a galvanized steel sheet having excellent ductility by specifying the chemical composition to be within a specific range and, in addition, specifying the volume ratios of retained austenite and martensite and the manufacturing conditions. Meanwhile, Japanese Unexamined Patent Application Publication No. 04-026744 discloses a galvanized steel sheet having excellent ductility by specifying the chemical composition to be within a specific range and, in addition, specifying the specific manufacturing condition. Japanese Unexamined Patent Application Publication No. 2007-182625 discloses a galvannealed steel sheet having excellent ductility by specifying the chemical composition to be within a specific range and, in addition, specifying the volume ratios of ferrite, bainitic ferrite, and retained austenite within specific ranges. Meanwhile, Japanese Unexamined Patent Application Publication No. 2000-212684 discloses a method of manufacturing a high strength cold rolled steel sheet which contains ferrite, bainite, and 3% or more of retained austenite and in which variations in elongation in the sheet width direction have been reduced.
However, in JP '022, JP '744 and JP '625, an improvement in ductility of the high strength thin steel sheet is the main purpose. Therefore, the hole expansion property is not taken into consideration. Also, in JP '684, only variations in the total elongation EL in the sheet width direction are described, and variations in mechanical properties due to the component composition and the production condition are not taken into consideration. As described above, in every technology, a high strength galvanized steel sheet having high ductility and high hole expansion property in combination and, in addition, having excellent stability of mechanical properties has not been obtained.
It could therefore be helpful to provide a method of manufacturing a high strength galvanized steel sheet having a tensile strength TS of 540 MPa or more and having excellent stability of mechanical properties and formability (high ductility and high hole expansion property) and, in addition, excellent coating appearance.